A disk drive is a digital data storage device that stores digital information within concentric tracks on a storage disk. In magnetic disk drive systems, the storage disk is coated with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. During operation of a disk drive, the disk is rotated about a central axis at a substantially constant rate. To read data from or write to the disk, a magnetic transducer is centered above a desired track of the disk while the disk is spinning. Writing is performed by delivering a write signal having a variable current to the transducer while the transducer is held close to the spinning track. The write signal creates a variable magnetic field at a gap portion of the transducer that induces magnetic polarity transitions into the desired track which are representative of the data being stored.
Reading is performed by sensing the magnetic polarity transitions on the rotating track with the transducer. As the disk spins below the transducer, the magnetic polarity transitions on the track present a varying magnetic field to the transducer. The transducer converts the varying magnetic field into an analog read signal that is then delivered to a read channel for appropriate processing. The read channel converts the analog read signal into a properly timed digital signal that can be recognized by a host computer system.
The transducer can include a single element, such as an inductive read/write element, for use in both reading or writing or it can include separate read and write elements. Transducers that include separate elements for reading and writing are known as "dual element heads" and usually include a magnetoresistive (MR) read element for performing the read function. Dual element heads are advantageous because each element of the transducer can be optimized to perform its particular function. For example, MR read elements are more sensitive to small variable magnetic fields than are inductive heads and thus can read much fainter signals from the disk surface. MR elements, however, are not capable of writing to the disk surface. Because MR elements are more sensitive, data can be more densely packed on the surface of the disk with no loss of read performance.
MR read elements generally include a strip of magnetoresistive material that is held between two magnetic shields. The resistance of the magnetoresistive material varies almost linearly with an applied magnetic field. During a read operation, the MR strip is held near a desired track, within the varying magnetic field caused by the magnetic transitions on the track A constant current is passed through the strip resulting in a variable voltage across the strip. By Ohm's law (i.e., V=I*R), the variable voltage is proportional to the varying resistance of the MR strip and hence is representative of the data stored within the desired track. The variable voltage signal (which is the analog read signal) is then processed and converted to digital form for use by the host.
There are many variables that can affect the read performance of a magnetic disk drive. One of the variables, for example, is the flying height of the transducer above the disk surface during the read. If the transducer is not within a specific flying height range during the read operation, the number of read errors that are created increases significantly. Another variable that affects read performance is the strength and position of the magnetic polarity transitions on the surface of the disk. If the transitions are weak or the data is not properly "centered" on the track, then the signal to noise ratio (SNR) of the analog read signal will be correspondingly low and poor read performance may result. Another variable that can affect the read performance of the disk drive is the presence of foreign particles or other aberrations on the surface of the disk that modulate the analog read signal when passed by the transducer. Signal distortions created by such particles are known as thermal asperities. When the transducer impacts a particle on the disk surface, the collision between the transducer and the particle generates a finite amount of heat that can change the read response of the transducer. For example, in a transducer having an MR read element, the heat generated by the collision changes the temperature of the MR strip which modulates the resistance of the strip. This resistance modulation adds an undesired baseline shift to the resulting analog read signal which can significantly increase the bit error rate of the disk drive.
Therefore, a need exists for a method and apparatus for recovering from the deleterious effects of thermal asperities. That is, there is a need for methods to accurately recover data from analog read signals having thermal asperity-type distortions.